Hydrogen on Demand is a (bullbleep) system that uses electricity from your alternator to make hydrogen from water, injects said hydrogen into your cylinders, and dramatically improves your fuel mileage (horse pucky).
Examples:
(broken link removed)
http://www.panacea-bocaf.org/hydrogenondemand.htm
There are legitimate ideas, wherein some sort of chemical reaction releases hydrogen when needed. These are seen as an alternative to pressurized tank storage of hydrogen gas. Note that this isn't a free lunch, since energy is needed to produce the chemical, or as in the example below, in the production and recycling of the metals in the aluminum alloy.
http://www.sciencedaily.com/releases/2008/02/080219133226.htm
Herein lies a problem of the hydrogen economy. Hydrogen is not a source of energy. There are no pools of liquid hydrogen that we can just suck up. It takes energy to produce hydrogen, so it is more accurate to say hydrogen is a storage medium, just another "battery". The energy must come from elsewhere. Perhaps we will get to the point where there is enough available electricity from nuclear power or renewables that we can produce sufficient hydrogen to get to a hydrogen economy. I am doubtful that this will truly come to pass. I think most uses will be satisfied by electrical power and motors once improved electrical storage comes along, be it better batteries, flywheels, or whatever. Perhaps hydrogen will be useful for applications where electrical storage cannot be dense enough, such as long haul freight, farm and construction equipment, etc.
The other big problem with hydrogen is distribution. As a gas, this tiniest of molecules leaks out of everything, permeating right through vessel walls. Liquefying helps, but that takes a lot of energy, and cryogenic hydrogen is so cold that it is really hard to handle. Imagine the expense of sufficiently insulated pipelines!
So, a hydrogen economy will require local production. Unless, of course, a chemical reaction method to make hydrogen on demand becomes practical.
Iceland may be the world's testbed. They have a lot of clean geothermal power. They are working on hydrogen fueling stations that tap this power and produce hydrogen on site. Then the switch to hydrogen powered cars. They also have the advantage of being less than 40,000 sq miles and only about 316,000 people, 1/3 of whom live in Reykjavík.
I'll stop now. The physics teacher in me just kicked in, but none of you are actually in my class, so I'll call it a day.